1. Field of the Invention
The present invention relates to semiconductor circuits and methods for driving the semiconductor circuits. The present invention relates to storage devices, display devices, and electronic devices.
2. Description of the Related Art
A field-effect transistor (FET) (hereinafter also referred to as a transistor) has three electrodes: a source, a gate, and a drain, and controls the flow of electrons or holes between the source and the drain by application of voltage to the gate. As a semiconductor used for an active layer, a Group IV element such as silicon or germanium, a Group III-V compound such as gallium arsenide, indium phosphide, or gallium nitride, a Group II-VI compound such as zinc sulfide or cadmium telluride, or the like can be used.
In recent years, FETs including an oxide such as zinc oxide or an indium gallium zinc oxide-based compound as a semiconductor have been reported (References 1 and 2). An FET including such an oxide semiconductor has comparatively high mobility and such a material has a wide bandgap of 3 eV or more; thus, application of a transistor including an oxide semiconductor to a display, a power device, or the like is discussed.
Transistors are roughly classified into two groups: enhancement transistors (normally off transistors) and depletion transistors (normally on transistors) according to their threshold voltages. In general, an enhancement transistor is off when a potential difference between a gate and a source is 0 V, whereas a depletion transistor is on when a potential difference between a gate and a source is 0 V.